1. Field of the Invention
The invention relates to a method of manufacturing capacitors integrated in a microelectronic structure at the surface of a semiconductor body, the plates of these capacitors being formed from two metal layers which are superimposed, are insulated at least in part from said semiconductor body and are mutually separated by a layer of an insulating material, the dielectric of the said capacitors and their lateral insulation being formed by means of islands and vertical partition walls, respectively, obtained by local anodic oxidation of the lower metal layer. The invention also relates to the device obtained by means of this method.
2. Description of the Prior Art
In order to form capacitors within an integrated microelectric structure, use is preferably made of the so-called thin film technique, the capacitors formed by means of this technique having a larger capacitance per unit surface area and considerably smaller resistances than the diffused junction capacitances that may also be utilized. Moreover, the capacitance of the thin film capacitors does not depend upon the applied voltage, as is the case with the capacitance of the semiconductor junctions.
In these capacitors, the insulator that has been used recently as a dielectric in fact is an oxide layer obtained from the metal constituting at least one of the plates.
In most cases, this dielectric is consequently alumina formed from the aluminum constituting the plates, the alumina being an oxide characterized by a high permittivity, which eliminates substantially entirely the risk of lack of insulation.
The method most frequently used for forming the dielectric is known as method of anodic oxidation, which consists in that a suitable voltage is applied between a layer of an anodically oxidizable metal and an electrode immersed in a suitably chosen electrolyte.
This solution, which is of particular importance when so-called "MOM" (Metal-Oxide-Metal) capacitors should be manufactured, because the dielectric layer can be formed in the first deposited metal layer, requires a suitable choice not only of the metal constituting the plates, most frequently aluminum, but also of the composition of the electrolyte employed. It is known indeed that given solutions used as electrolyte, such as, for example, oxalic acid and aethylene-glycol, lead to the formation of a porous layer of alumina during the anodization of a layer of aluminum, while other solutions, especially on the basis of tartaric acid, yield a tight layer of alumina.
It goes without saying that, in order to avoid the diffusion of the metal constituting the plates of the capacitor through its dielectric, the latter must advantageously be constituted by a tight oxide layer.
Generally, the metal plates of the capacitors are formed during the process of shaping the connection networks covering the surface of the semiconductor body carrying the integrated micro-structure.
If the latter comprises a plurality of capacitors, which may be identical, at the same level, its manufacture has consequently become more complex. In this case, therefore, the problems of insulation between the said capacitors and the problems of contacting should be taken into consideration, which problems are connected inter alia with the space required and can be solved with greater difficulty as the surface of the said integrated micro-structure is smaller.
A solution consists in that for the assembly use is made of a plate formed from a first metal layer deposited on an insulating layer covering the surface of the semiconductor body which carries the integrated micro-structure, the other plate of each of the said capacitors being obtained from a second metal layer, and in that at least the said first metal layer is subdivided into different parts of configurations chosen beforehand by means of insulating transverse partition walls formed from its surface.
Said partition walls may be obtained by means of a groove provided in the metal layer(s) and may be filled with silicon oxide, but they are preferably obtained by local anodic oxidation of the first metal layer which also serves to obtain a plate of the capacitors and generally to obtain connection networks.
After the capacitors and the connection networks have been formed at the surface of the semiconductor body, the latter has finally to be subjected to a step of polishing its back surface opposite the so-called active surface carrying the active or passive elements of the integrated structure.
This polishing step serves to obtain the desired thickness of the semiconductor body and it is effected while using the active surface and the layer covering it as a reference surface.
In the modern structures, under the influence of the pressure exerted on the reference surface, the dielectric local islands separating the plates of the capacitors as well as the vertical partition walls insulating said capacitors from each other are pressed slightly into the metal layer in which they have been formed and may then be damaged.
This phenomenon, which becomes manifest especially at the periphery of the said islands and/or the said partition walls, may produce short circuits between the plates of the same capacitors or between the plates of adjacent capacitors.
A solution used to obviate this disadvantage consists in that the dielectric layer or the insulating partition walls are covered with a supplementary insulating layer, which consequently causes deposition and photo-etching steps to be added to the existing mode of operation and increases the complexity of the method.